4GB PC3-12800 DDR3 ECC UDIMM Server Memory RAM for HP Z400 Workstation 6 DIMM | eBay
CMS 12GB (3X4GB) DDR3 10600 1333MHZ ECC Non Registered DIMM Memory Ram Upgrade Compatible with HP/Compaq® Workstation Z200, Z210, Z400, Z600, Z800 for Server Only - B81 at Amazon.com
12GB KIT (6 x 2GB) for HP-Compaq Z Workstation Series Z400 (6 DIMM Slots). DIMM DDR3 ECC Unbuffered PC3-10600E 1333MHz Server Ram Memory. Genuine A-Tech Brand. at Amazon.com
HP Z400 Intel Xeon QUAD Core W3520@2.67GHz 8GB RAM 500GB HD Workstation PC Win10 6734961703543 | eBay
HP Z400 Workstation - Memory | HP® Customer Support